Field
The present disclosure relates to a damper.
Related Art
Conventionally, a motorcycle includes dampers to absorb impact from the ground so as to improve riding comfort. The dampers include, for example, a cylinder and a piston slidably fitted in the cylinder. Sliding of the piston in the cylinder causes oil to flow, and a flow of the oil is guided into a damping force generator disposed inside or outside of the cylinder. Thus, damping force is generated.
When the piston slides in the cylinder, it is necessary to realize both of two properties. One of them is a sliding property of the piston in the cylinder. The other is a sealing property in sealing up oil in two oil chambers in the cylinder that are defined by the piston so as to prevent the oil from leaking from a gap between the piston and the cylinder so that the oil does not flow between the two oil chambers through the gap. For this purpose, a piston ring is attached to an outer circumference of the piston. A material such as resin is used for the piston ring to secure these two properties.
In some of the dampers, the cylinder has a double-tube configuration including an inner tube and an outer tube. In the double-tube configuration, communication holes that allow oil to flow between the inner tube and the outer tube are formed in the inner tube. Consequently, when sliding of the piston in the inner tube is repeated, the piston ring, which slides on a portion of the inner tube that has the communication holes, may be unfortunately damaged.
In view of this, in the damper with the double-tube configuration, it is necessary to stop the piston itself, to which the piston ring is attached, before the portion of the inner tube that has the communication holes. Thus, the piston does not slide on the portion of the inner tube that has the communication holes.
For example, Japanese Unexamined Patent Application Publication No. 2011-12806 discloses a damper including communication holes in a vicinity of a lower end of an inner tube of a cylinder having the double-tube configuration. Specifically, when a rebound spring is compressed to have the minimum length, the piston is displaced to the lowermost position and stopped there. The communication holes are formed further on a lower end side of a lowermost position of the piston. Thus, the piston ring attached to the outer circumference of the piston is also stopped before the portion of the inner tube that has the communication holes. This prevents the piston ring from being damaged.
This configuration of the damper for preventing damage of the piston ring is not limited to the damper disclosed in Japanese Unexamined Patent Application Publication No. 2011-12806. For example, in a damper for a motorcycle that travels on a highly bumpy ground in a situation such as a motocross, a stopper is disposed at a predetermined distance from a lower end of the piston, and this stopper stops the piston at a time of maximum extension of the damper. Moreover, a rebound spring and a rubber disposed on a lower end side of the stopper absorb impact at the time of maximum extension of the damper.
In this damper, communication holes in the inner tube of the double-tube configuration are formed on a lower end side of the position of the stopper that is stopped at the time of maximum extension of the damper. With this configuration, the piston ring attached to the outer circumference of the piston is stopped before the portion of the inner tube that has the communication holes so as to prevent the piston ring from being damaged.
The damper disclosed in Japanese Unexamined Patent Application Publication No. 2011-12806 includes a cylinder and a piston slidably fitted in the cylinder. The piston slides in the cylinder to cause oil to flow, and a flow of the oil is guided into a damping force generator outside of the cylinder so as to generate damping force.
In a case of the piston in this damper as well, in order to realize the sliding property and the sealing property with respect to the cylinder, a piston ring made of resin is attached to an outer circumference of the piston. The cylinder of the damper also includes an annular chamber between an outer tube and the inner tube. An inside of the inner tube in the damper is defined into a piston-side oil chamber and a rod-side oil chamber by the piston. Oil flows into and from the piston-side oil chamber and the rod-side oil chamber through the annular oil chamber.
This damper includes the damping force generator outside of the cylinder. Consequently, for example, when the piston slides in the cylinder to cause the oil to flow, the oil flows in the rod-side oil chamber, the annular oil chamber, the damping force generator, and the piston-side oil chamber in this order at a rebound stroke, and the oil flows in the piston-side oil chamber, the damping force generator, the annular oil chamber, and the rod-side oil chamber in this order at a compression stroke. In any case, in the damper disclosed in Japanese Unexamined Patent Application Publication No. 2011-12806, the cylinder and the piston function as a pump mechanism to send out the oil.
Detailed description will now be made on a configuration in which the stopper is disposed at a predetermined position on a lower end side of the piston in the damper in which the cylinder and the piston function as the pump mechanism to send out the oil, for example.
FIG. 9 is a partially enlarged, vertically cross-sectional view of a piston 320 and components surrounding it of a pump mechanism 300 in a conventional damper. In FIG. 9, the pump mechanism 300 is extended to the maximum extent.
As illustrated in FIG. 9, the pump mechanism 300 mainly includes such components as a cylinder 311, a piston rod 312, the piston 320, and a rod guide 314.
The cylinder 311 is formed of a double tube including an outer tube 318 and an inner tube 319. Upper ends of the outer tube 318 and the inner tube 319 are closed by a cap (not illustrated). Lower ends of the outer tube 318 and the inner tube 319 are opened. The rod guide 314 is secured in an opening of the outer tube 318. The piston rod 312 slidably penetrates the rod guide 314.
An upper end of the piston rod 312 is attached to the piston 320. A rod end 330 is disposed inside of the upper end of the piston rod 312 so as to fill an opening of the hollow piston rod 312. A lower end of the piston rod 312 is extended outside of the cylinder 311. The piston rod 312 and the piston 320 are inserted together in the inner tube 319. The piston 320 slides on an inner circumference 319b of the inner tube 319. A piston ring 320a made of resin is attached to an outer circumference of the piston 320.
Here, it is noted that “upper end side” refers to an upper side in a direction of a common axis of the cylinder 311 and the piston rod 312, which are coaxial with each other (the same applies below). “Lower end side” refers to a lower side in the direction of the common axis of the cylinder 311 and the piston rod 312, which are coaxial with each other (the same applies below). A vertical direction along the common axis of the cylinder 311 and the piston rod 312, which are coaxial with each other, in FIG. 9 is referred to as an axial direction (the same applies below).
As illustrated in FIG. 9, a stopper 322 is attached to the piston rod 312 on a lower end side of the piston 320 with a collar 315 interposed between the stopper 322 and the piston 320. The piston rod 312 penetrates a center of the stopper 322, and the stopper 322 is secured on the piston rod 312.
The rod guide 314 is, for example, secured to an inner circumference of the lower end of the outer tube 318 in a fluid tight manner. As illustrated in FIG. 9, a stepped portion 314a is formed on an inner circumference of the rod guide 314. This stepped portion 314a is formed of a flat surface perpendicular to the axial direction and extends in a circumferential direction.
An annular stopper receiver 324 is attached to the stepped portion 314a. At a rebound stroke, a lower end 322a of the stopper 322 is brought into contact with this stopper receiver 324 to stop displacement of the piston 320 and the piston rod 312 toward the lower end side. This configuration limits a maximum rebound stroke of the pump mechanism 300. At this time, the collar 315 is interposed between the piston 320 and the stopper 322. Since the collar 315 has a constant length, the position of the piston 320 at a maximum end of the rebound stroke is determined. A lower end 319c of the inner tube 319 is, for example, in contact with an upper end side of the stopper receiver 324.
A support 331 to support an annular rebound rubber 323 is disposed on a lower end side of the stopper receiver 324 and on the inner circumferential side of the rod guide 314. This support 331 is formed of an annular member. At a maximum end of the rebound stroke of the pump mechanism 300, the rebound rubber 323 is brought into contact with the stopper 322 so as to absorb impact at a time of maximum extension of the pump mechanism 300.
An oil chamber 314c is formed around the piston rod 312 and on the inner circumferential side of the support 331 and the rebound rubber 323. A rebound spring 326 is disposed in the oil chamber 314c, and the piston rod 312 penetrates the rebound spring 326. At the maximum end of the rebound stroke of the pump mechanism 300, the rebound spring 326 is brought into contact with the stopper 322 so as to absorb the impact at the time of maximum extension of the pump mechanism 300. In this configuration, the rebound rubber 323 and the rebound spring 326 exclusively absorb the impact at the time of maximum extension of the pump mechanism 300. This, however, should not be construed in a limiting sense. When the stopper receiver 324 is not provided, the rebound rubber 323 and the rebound spring 326 may restrict displacement of the stopper 322 toward the lower end side so as to limit the maximum rebound stroke of the pump mechanism 300. It is noted that the oil chamber 314c is filled with oil as the fluid.
An oil seal 325 is attached to an inner portion of the rod guide 314 in such a manner that the piston rod 312 is slidable in the fluid tight manner. A collar 316 is disposed between the piston rod 312 and the rod guide 314 on a lower end side of the oil seal 325.
An oil chamber 321 in the inner tube 319 is defined into a piston-side oil chamber 321a and a rod-side oil chamber 321b by the piston 320. It is noted that the oil chamber 314c functions as part of the rod-side oil chamber 321b. When the lower end 322a of the stopper 322 is brought into contact with the rebound rubber 323, the oil chamber 314c is closed.
When closed, the oil chamber 314c does not communicate with the rod-side oil chamber 321b between the piston 320 and the stopper 322 and with an annular oil chamber 327, described later. That is, when closed, the oil chamber 314c is sealed. When not closed, the oil chamber 314c communicates with the rod-side oil chamber 321b between the piston 320 and the stopper 322 and with the annular oil chamber 327, described later.
The annular oil chamber 327 is formed between the outer tube 318 and the inner tube 319, which constitute the cylinder 311. The annular oil chamber 327 is filled with the oil. A communication hole 319a is formed in the inner tube 319 on the lower end side. Even at the maximum end of the rebound stroke of the pump mechanism 300, the communication hole 319a is at a position on the lower end side of the piston 320. For example, a plurality of communication holes 319a are formed in the circumferential direction.
As illustrated in FIG. 9, the annular oil chamber 327 communicates with the rod-side oil chamber 321b through the communication holes 319a For example, the annular oil chamber 327 communicates with a damping force generator (not illustrated) through an oil passage (not illustrated) in the cap (not illustrated). For example, the piston-side oil chamber 321a communicates with the damping force generator (not illustrated) through an oil passage (not illustrated) in the cap (not illustrated).
Description will now be made on a flow of the oil in the rod-side oil chamber 321b of the pump mechanism 300 in the conventional damper of the configuration described above.
FIG. 10 is a partially enlarged, vertically cross-sectional view of the piston 320 and the components surrounding it when the piston 320 slides in the inner tube 319 in the pump mechanism 300 of the conventional damper. For ease of description, functions in the compression stroke and the rebound stroke are described using one drawing. In FIG. 10, an oil flow direction at the compression stroke is indicated with solid lines while an oil flow direction at the rebound stroke is indicated with dashed lines. It is noted that at the compression stroke, the piston 320 is displaced toward the upper end side, and that at the rebound stroke, the piston 320 is displaced toward the lower end side.
FIG. 11 is a graph illustrating a relationship between passage resistance and a stroke of the conventional damper. The passage resistance is caused in a gap 328 between the stopper 322 and the inner tube 319 when the piston 320 slides in the inner tube 319 in the pump mechanism 300 of the conventional damper. The oil flows between a side portion 322b of the stopper 322 and the inner circumference of the inner tube 319. Thus, the passage resistance is generated in the gap 328.
First, a rebound stroke will be described.
At the rebound stroke, the piston 320 is displaced toward the lower end side. At this time, oil in the rod-side oil chamber 321b flows through the communication holes 319a on a lower end side of the inner tube 319 to the annular oil chamber 327 between the outer tube 318 and the inner tube 319 (see dashed-line arrows in FIG. 10). The oil, which has flowed into the annular oil chamber 327, is guided to the damping force generator (not illustrated). The flow of the oil is damped by the damping force generator (not illustrated), and the oil flows through the oil passage (not illustrated) in the cap (not illustrated) into the piston-side oil chamber 321a. 
When the stopper 322 is displaced in the inner tube 319 toward the lower end side and reaches the communication holes 319a of the inner tube 319, volume of the oil chamber 314c on a lower end side of the stopper 322 is gradually compressed to make the oil on the lower end side of the stopper 322 in the rod-side oil chamber 321b move toward the upper end side, pass the communication holes 319a, and flow into the annular oil chamber 327.
The oil on the lower end side of the stopper 322 in the rod-side oil chamber 321b, which flows toward the communication holes 319a, flows in the gap 328 between the side portion 322b of the stopper 322 and the inner circumference 319b of the inner tube 319. The flow of the oil in the gap 328 generates the passage resistance.
As illustrated in FIG. 11, the passage resistance in the gap 328 increases from a position (that corresponds to point “h” in FIG. 11) in which the lower end 322a of the stopper 322 reaches an upper end U of the communication holes 319a to a position (that corresponds to point “i” in FIG. 11) in which an upper end 322c of the stopper 322 reaches a lower end D of the communication holes 319a After the upper end 322c of the stopper 322 passes the lower end D of the communication holes 319a, the oil flows only in the gap 328. Consequently, the passage resistance in the gap 328 increases until the maximum point “i” and then becomes constant (between point “i” and point “j” in FIG. 11). At the maximum end of the rebound stroke of the pump mechanism 300, not only the rebound spring 326 and the rebound rubber 323 absorb impact of the stopper 322 on the stopper receiver 324 but also the passage resistance in the gap 328 further absorbs the impact.
Next, a compression stroke will be described.
In this description, assume that the compression stroke starts from the maximum end of the rebound stroke of the pump mechanism 300 illustrated in FIG. 9.
At the compression stroke, the piston 320 is displaced toward the upper end side. At this time, oil in the piston-side oil chamber 321a is guided through the oil passage (not illustrated) in the cap (not illustrated) toward the damping force generator (not illustrated). The flow of the oil is damped by the damping force generator (not illustrated), and the oil flows into the annular oil chamber 327. The oil, which has flowed into the annular oil chamber 327, flows through the communication holes 319a on the lower end side of the inner tube 319 into the rod-side oil chamber 321b. 
Before the upper end 322c of the stopper 322 approaches the lower end D of the communication holes 319a, the oil, which has flowed from the annular oil chamber 327 into the rod-side oil chamber 321b through the communication holes 319a, flows only in the gap 328 toward the lower end side. Consequently, as illustrated in FIG. 11, constant passage resistance is generated in the gap 328 (between point “j” and point “i” in FIG. 11). Therefore, generation of the passage resistance degrades operability of the piston 320 in the pump mechanism 300 at a start of displacement of the piston 320 that shifts from the rebound stroke to the compression stroke.
As illustrated in FIG. 11, this passage resistance decreases from the position (that corresponds to point “i” in FIG. 11) in which the upper end 322c of the stopper 322 reaches the lower end D of the communication holes 319a to the position (that corresponds to point “h” in FIG. 11) in which the lower end 322a of the stopper 322 reaches the upper end U of the communication holes 319a. After the lower end 322a of the stopper 322 passes the upper end U of the communication holes 319a, the oil does not flow in the gap 328. The reason is that a distance between the piston 320 and the stopper 322 is equal and constant to the length of the collar 315, and an oil chamber between the piston 320 and the stopper 322 in the rod-side oil chamber 321b does not change in volume, and basically, no flow of the oil is generated. Therefore, the passage resistance is not generated in the gap 328 and is constant at 0 (between point “h” and point “g” in FIG. 11).
It is noted that after the lower end 322a of the stopper 322 passes the upper end U of the communication holes 319a, the oil from the communication holes 319a directly flows toward the lower end side of the stopper 322 in the rod-side oil chamber 321b. 